1. Field of the Invention
This invention relates to optical mensuration devices in general, and in particular to an improved method and apparatus for the optical mensuration of the surface shape of a three-dimensional object.
2. Brief Description of the Prior Art
Numerous mensuration systems exist in the prior art for sensing the locations of surface points on three-dimensional solid objects in relation to a predefined fixed reference frame or coordinate system for input into an application system, such as a computer or other device for measurement or analysis. For example, one type of mensuration system that can be used to determine the location of a single point on the surface of an object includes the use of a narrow projected beam of light to illuminate a tiny area or spot on the surface of the object. A lens in the system is positioned on an optical axis oblique to the axis of the projected beam and is used to focus the reflected light from the illuminated spot onto a photoelectric sensor or onto a linear array of sensors. Since the optical axis of the lens and sensor assembly in that type of system is not coincident with the axis of the projected beam, the position of the image of the illuminated spot on the sensor will depend on the location of the particular illuminated surface point with respect to the illuminating beam. Therefore, the location of the illuminated point with respect to the predetermined reference frame can be determined by computing the distance of the illuminated surface point from the origin of the light beam which, of course, is known. Examples of such point illumination optical mensuration systems are found in the following U.S. Pat. Nos. 4,660,970; 4,701,049; 4,705,395; 4,709,156; 4,733,969; 4,743,770; 4,753,528; 4,761,072; 4,764,016; 4,782,239; and 4,825,091.
Of course, to determine the overall shape of an object, numerous individual surface points, along with their respective locations, must be measured and recorded. Such optical measurement of multiple surface points of an object is typically accomplished by mounting the beam projector on a moveable scanning head capable of being moved from point-to-point with very high precision, such as the type commonly found on numerically controlled milling machines. By precisely moving the beam projector mounted on the scanning head in a raster-like scanning pattern, it is possible to measure the surface shape of the object being scanned by measuring the individual locations of surface points individually illuminated by the point-like scanning beam as it is scanned over the object's surface. Alternatively, the object itself can be moved while the scanning head remains stationary. One disadvantage of this type of system is that only one side of the object may be scanned at any one time, since other sides of the object are hidden by the side being scanned. Scanning of these hidden sides can only be accomplished by relocating either the scanning head or the object to expos the previously hidden surfaces to the scanning beam. Obviously, such a relocation requires time and precision equipment to keep track of the changed position of the scanning head, or the object in relation to the fixed reference frame so that the new surface data will correspond to the previously obtained surface data. Helical or three-dimensional scanning heads solve this problem by allowing the entire object to be scanned at once. However, such helical systems are relatively expensive, since they require complex mechanical apparatus to move the scanning head around the object in three-dimensions.
Regardless of the scanning method used, however, deep holes, overhangs, undercuts, and surfaces nearly parallel to the axis of the scanning beam reduce the accuracy of the system, since it is difficult to accurately measure these points, if they can even be illuminated by the scanning beam at all. For example, such systems cannot completely scan the inside, outside, and handle details of a coffee cup without requiring the scanning apparatus to be relocated or the object to be reoriented so that the inside surfaces or other surfaces previously hidden from the scanning beam can be illuminated by the beam, thus measured and recorded. As discussed earlier, such re-locations or re-orientations have the disadvantage of having to recalibrate the scanning apparatus, or otherwise recorrelate the new surface points with respect to the original coordinate system. Moreover, even if such relocations or reorientations are not required, such as in the case of a helical scanning apparatus, there is still a severe loss of accuracy when scanning near the top or bottom of a rounded object, unless the scanning head and detector are relocated to better illuminate and detect such points. Furthermore, these types of systems are not very portable or adaptable since they require high precision electro-mechanical or other apparatus to accurately move the scanning heads (or the object) and define their positions in relation to the predetermined reference frames. Therefore, all these prior art scanning systems will usually require some type of relocation of the scanning apparatus or reorientation of the object to completely measure and record all of the surface details.
A variant of the above-described systems projects a thin beam of light in a single plane which, of course, is incident as a line, as opposed to a point, on the surface of the object being scanned. The intersection of this plane of light with the object's surface thus forms a brightly illuminated contour line. A two-dimensional electronic video camera or similar device whose optical axis is not coincident with the axis of the illuminating beam, detects the image of this contour line. Again, since the optical axis of the camera is not coincident with the axis of the illuminating light beam, it views the contour line from an oblique angle, thus allowing location of the contour line to be precisely determined in relation to the known position of the beam projector. Examples of inventions using this type of system are found in the following U.S. Pat. Nos. 4,821,200; 4,701,047; 4,705,401; 4,737,032; 4,745,290; 4,794,262; 4,821,200; 4,743,771; and 4,822,163.
To measure more than one contour line of an object, either the measuring apparatus or the object is panned along (or rotated about) an axis through the object. While these line scanning devices share similar drawbacks with the point scanning devices previously described, they do operate much faster, gathering a larger number of sample points during a given scanning interval. Unfortunately, the accuracy of each surface sample point is limited by the relatively low resolution of the two-dimensional charge coupled device (CCD) sensors found in most video cameras, which is typically in the range of 1 part in 512. Even worse, these systems still suffer the disadvantages of the point scanning systems in that either the scanning head or the object must be relocated or re-oriented to completely and accurately record all of the surface details of an object.
Still other mensuration systems track the positions of specific points in three-dimensional space by using small radiating emitters which move relative to fixed receiving sensors, or vice versa. Such radiation emitters may take the form of sound, light, or nutating magnetic fields. Another mensuration system uses a pair of video cameras plus a computer to calculate the position of homologous points in the pair of stereographic video images. See, for example, U.S. Pat. Nos. 4,836,778 and 4,829,373. The points tracked by this system may be passive reflectors or active light sources. The latter simplifies finding and distinguishing the points.
Additional prior art relevant to this patent application are found in the following references:
Burton, R. P.; Sutherland, I. E.; "Twinkle Box--a three dimensional computer input device", National Computer Conference, AFIPS Proceedings, v 43, 1974, p 513-520; PA0 Fischer, P.; Mesqui, F.; Kaeser, F.; "stereometric measurement system for quantification of object forms", SPIE Biostereometrics 602, 1985, p 52-57; PA0 Fuchs, H.; Duran, J.; Johnson, B.; "Acquisition and Modeling of Human Body Form Data", Proc. SPIE, v 166, 1978, p 94-102; PA0 Macellari, V.; "A Computer Peripheral Remote Sensing Device for 3-Dimensional; Monitoring of Human Motion", Med. & Biol. Eng. & Comput., 21, 1983, p 311-318; PA0 Mesqui, F.; Kaeser, F.; Fischer, P.; "real-time, noninvasive recording and 3-d display of the functional movements of an arbitrary mandible point", SPIE Biostereometrics 602, 1985, p 77-84; PA0 Yamashita, Y.; Suzuki, N.; Oshima, M.; "Three-Dimensional Stereometric Measurement System Using Optical Scanners, Cylindrical Lenses, and Line Sensors", Proc. SPIE, v. 361, 1983, p. 67-73.
In particular, the paper by Fuchs, et al, (1978) describes a basic method of tracking a light source in three-dimensional space. The method is based on using three or more one-dimensional sensors, each consisting of a cylindrical lens and a linear array of photodetectors, such as charge coupled devices (CCDs), to determine the location of the currently radiating source.
Numerous other methods have been devised and patented for determining the position of a point along a line, within a plane, or in three-dimensional space. Devices employing these methods include photographic camera rangefinders, tablet digitizers, coordinate measuring machines, and surveying tools. Some exploit sound, magnetic fields, or mechanical apparatus for mensuration, and there are other devices employing x-rays, nuclear magnetic resonance, radar, sonar, and holography to sense the shapes of objects.
Unfortunately, each of the above mensuration systems has its own set of drawbacks, which include high cost, poor accuracy, poor resolution, awkward or difficult use, limitations on geometrical complexity, excessive numerical computation, or slow measurement speed. Experience has shown that no single prior art system best suits all three-dimensional measurement applications. For example, there is no existing mensuration device that can perform even straightforward anatomical measurements of a person without significant drawbacks.
Thus, there remains a need for a non-contact, three-dimensional optical mensuration system which is capable of accurate, speedy, convenient, and inexpensive sensing of three-dimensional geometric shapes or objects. Ideally, the scanning head of such an improved system should be hand-held to allow the operator to easily move the scanning beam over some of the more complex surface details of the object while dispensing with the need for the expensive, cumbersome, and high precision scanning head positioning apparatus currently required. Such a hand-held scanner must also provide the accuracy and precision associated with currently available optical mensuration systems, that is, it must be able to accurately measure and precisely locate the surface details of the object in relation to the predetermined reference frame.